investigation: Conditional GCM staleness support

[bwplotka]

To emit NaN:

Clone this repo and:

export GCM_SECRET=<gcm secret>
go test ./google/internal/promqle2etest/... -run TestExportGCM_Staleness
go test ./google/internal/promqle2etest/... -run TestExportGCM_StalenessCounter

Or (as a maintainer) push to this branch or checkout this branch and create a similar PR with your changes.

To play with PromQL + staleness

# Configure the test as you wish
vim ./promql/promqltest/testdata/staleness.test
# Run the staleness.test (this PR hardcodes this test to run staleness.test only
go test ./promql/... -run TestEvaluations

cc @realschwa